The present invention relates to the treatment of 96% silica glass and particularly to a process for treating porous 96% silica glass to increase the annealing point thereof.
Glasses of the kind referred to as 96% silica glasses, and methods for manufacturing the same, are described in U.S. Pat. No. 2,106,744 to Hood et al. In accordance with such methods, alkali borosilicate glasses of specified composition are subjected to a phase separation heat treatment to separate the glass into a silica-rich phase and a phase rich in alkali and boron. This latter phase is then removed by leaching to provide a microporous glass body comprising a very high percentage of silica (typically at least about 94% by weight) and minor amounts of residual alkali and boron.
Glass produced by this process is referred to as 96% silica glass without regard to the exact silica content thereof, and may be used either in the porous state or after consolidation to the non-porous state by heating. In porous form, the glass may be used to support catalysts and enzymes, and as a chromatographic medium. Consolidated 96% silica glass is used to provide refractory glass articles such as crucibles and envelopes for arc lamps.
The annealing point of 96% silica glass depends on the composition of the glass, and is reduced by residual alkali and boron in the glass as well as fluxing constituents such as hydroxyl groups and halogen groups. Transparent 96% silica glass having a high annealing point can be provided by subjecting porous 96% silica glass to supplemental treatments such as described in U.S. Pat. Nos. 3,113,008 and 3,113,855 to Elmer. One such treatment comprises reheating and releaching to remove further alkali and boron from the glass, a procedure which is both inefficient and increasingly expensive. Another such procedure disadvantageously involves the use of a prolonged treatment in an ammonia-containing gas stream.
It is known that the annealing point of 96% silica glass is raised if a carbon phase is present in the glass. U.S. Pat. Nos. 3,775,078 to Elmer et al. and 3,813,232 to Forker et al. describe the use of liquid treating media for producing such a carbon phase, while U.S. Pat. No. 3,475,211 to Hasegawa et al. describes a vapor deposition method to provide a similar result.
Products resulting from the above carbon impregnation methods, although quite refractory, are electrically conducting, black in color, non-porous, and opaque. There are many applications for which a transparent, electrically insulating, high-annealing-point glass would be preferred.
It is a principal object of the present invention to provide a convenient process for increasing the annealing point of 96% silica glass without unduly affecting the transparency of the product or producing an electrically conducting glass.
Other objects and advantages of the invention will become apparent from the following description thereof.